Generally, substrates, for example of high-speed steel and cemented carbide, having a hard film such as of titanium nitride or titanium aluminum nitride formed on the surface thereof by physical vapor deposition method (hereinafter, referred to as PVD method), chemical vapor deposition method (hereinafter, referred to as CVD method), or the like have been used as cutting tools and sliding parts, for which superior wear resistance and sliding property are demanded.
In particular, in case of application as a cutting tool that demands high wear and heat resistance (oxidation resistance at high temperature) from the hard film, titanium aluminum nitride (TiAlN) that is superior in both of these properties has been used widely as a coating material for applications such as cemented carbide tools, the edge of which becomes heated to high temperature during machining. The reason for such advantageous properties of TiAlN is that the TiAlN film is improved in heat resistance by the action of aluminum contained in the film and exhibits excellent wear and heat resistance consistently up to a high temperature of approximately 800° C. Various TiAlN's different in the compositions of Ti and Al have been used as the TiAlN films, but most of them have a composition in which they are is superior in both properties, that is, a Ti:Al atomic ratio in the range of 50:50 to 25:75.
Meanwhile, the edge of a cutting tool or the like is occasionally heated to a temperature of 1,000° C. or higher during machining. Under such circumstance, because the TiAlN film alone is not sufficiently effective as it is in ensuring heat resistance, it is often practiced to form an alumina layer additionally on a TiAlN film for ensuring heat resistance, as disclosed, for example, in U.S. Pat. No. 5,879,823.
Alumina has various crystal structures, depending on temperature, but all of the crystal structures are in the thermally metastable state. However, in the case of a cutting tool, the temperature of the edge fluctuates in a wide range from room temperature to 1,000° C. or higher during machining, which causes transformation in the crystal structure of alumina and consequently leads to problems such as the cracking and delamination of film. However, alumina in the alpha crystal structure that is formed by CVD at a higher substrate temperature of 1,000° C. or more, if once formed, retains its thermally stable structure, independently of the temperature thereafter. Thus, deposition of an alumina film in the alpha crystal structure is regarded as an effective means of providing heat resistance to cutting tools and others.
However as described above, it is necessary to heat the substrate to 1,000° C. or higher to form alumina in the alpha crystal structure, which restricts the kind of compatible substrates. It is because some substrates soften and lose its favorable properties as a substrate for wear-resistant products, depending on its kind, when exposed to a high temperature of 1,000° C. or higher. Even high heat-resistant substrates including cemented carbides also exhibit problems such as deformation, when exposed to such a high temperature. In addition, practical temperature range for use of the hard films such as TiAlN film that are formed on substrate as an wear-resisting film is generally, approximately 800° C. at the highest, and such films may degenerate and lead to deterioration in wear resistance when exposed to a high temperature of 1,000° C. or higher
For overcoming such problems, U.S. Pat. No. 5,310,607 reported that it was possible to obtain a (Al,Cr)2O3 composite crystal having a hardness as high as that of the alumina described above in a lower temperature range of 500° C. or lower. However if the work material contains iron as the principal component, Cr existing on the surface of the composite crystal film often reacts with iron in the work material chemically during machining, leading to enhanced consumption of the film and reduction of the lifetime.
Separately, O. Zywitzki, G. Hoetzsch, et al. reported in “Surf. Coat. Technol.” (86-87, 1996, p. 640-647) that it was possible to form an aluminum oxide film in the alpha crystal structure at 750° C. by performing reactive sputtering by using a pulse power supply having a high output (11 to 17 kW). However, it is inevitable to expand the capacity of pulse power supply for obtaining aluminum oxide in the alpha crystal structure by this method.
As a method overcoming such problems, Japanese unexamined patent publication No. 2002-53946 discloses a method of forming an oxide film having a film thickness of at least 0.005 μm in the corundum structure (alpha crystal structure) having a lattice parameter of 4.779 Å or more and 5.000 Å or less as an underlayer and forming an alumina film in the alpha crystal structure on the underlayer. The patent application above shows that the component for the oxide film is preferably Cr2O3, (Fe,Cr)2O3 or (Al,Cr)2O3; more preferably (Fex,Cr(1-x))2O3 (wherein, 0≦x≦0.54) when it is (Fe,Cr)2O3 and (Aly,Cr(1-y))2O3 (wherein, 0≦y≦0.90) when it is (Al,Cr)2O3.
It also indicates that it is effective to form a film of a composite nitride containing Al and one or more elements selected from the group consisting of Ti, Cr, and V as a hard film, a film of (Alz,Cr(1-z))N (wherein, 0≦z≦0.90) as an intermediate layer thereon which forms an oxide film having the corundum structure (alpha crystal structure) by further oxidizing the film above, and then form alumina in the alpha crystal structure on the oxide film.
The inventors proposed various methods for forming an alumina film mainly in alpha crystal structure on a hard film (for example, Japanese Patent Application No. 2002-233848).
However, when the methods proposed by the inventors are applied to various kinds of substrates other than hard films, it was not possible to obtain a quasi-alpha-alumina film, depending on the kind of substrate. In addition when an alumina film is formed at a temperature of about 700° C. in the relatively lower temperature range on the hard film above, which usually allows generation of the quasi-alpha-alumina film, the ratio of the y to alpha crystal structure increased in the alumina film obtained. Experimental results confirming the description above will be described below in detail.
First, the following three substrates (1) to (3) were prepared and the following experiments A and B were performed.
(1) Si wafer
(2) Cemented carbide substrate (12.7 mm×12.7 mm×5 mm) polished to mirror surface (Ra: approximately 0.02 μm) having a 2- to 3-μm TiAlN (Ti0.55Al0.45N) hard film formed thereon by the arc ion-plating method (hereinafter, referred to as AIP method) as a hard film
(3) cemented carbide substrate treated similarly to the substrate (2) having a 2- to 3-μm CrN film formed thereon by the AIP method as a hard film.
<Experiment A>
Each substrate was first oxidized, and then, an alumina film was formed thereon. The substrate was oxidized and the alumina film was formed in the vacuum deposition device shown in FIG. 1 (AIP-S40 hybrid coater machine, manufactured by Kobe Steel) equipped with an AIP cathode (indicated by No. 7 in FIG. 1 below), a magnetron-sputtering cathode, a heater heating mechanism, a substrate-rotating mechanism, and others.
The substrate was oxidized as follows: A sample (substrate) 2 was connected to each planetary revolving jig 4 on the revolving table 3 in the device 1; the device was evacuated almost to the vacuum state; and the sample was heated to 750° C. with a heater 5 placed in the center of the device and heaters 5 placed on the internal side wall 2 of the device. After the sample is heated to a particular temperature, an oxygen gas was introduced into the device 1 to a pressure of 0.75 Pa at a flow rate of 300 sccm, and the sample was oxidized while heated in the same condition for 20 minutes.
An alumina film was formed as follows: An alumina film of approximately 2 μm in thickness was formed by the reactive sputtering method, specifically by applying a pulse DC power of approximately 2.5 kW respectively to the two sputtering cathodes 6 having an aluminum target shown in FIG. 1, at a substrate temperature similar to that in the oxidation step in an argon and oxygen environment. The alumina film was formed at a discharge condition in a so-called transition mode, i.e., by controlling the discharge voltage and the flow rate ratio of argon/oxygen by plasma emission spectroscopy.
The crystal structure of the alumina film formed on the outmost layer was identified by analyzing the surface of the thin film thus formed with an X-ray diffractometer. FIG. 2 shows the thin-film X-ray diffraction pattern of the alumina film formed when the substrate (2) (TiAlN film) was used; FIG. 3, when the substrate (3) (CrN film) was used; and FIG. 4, when the substrate (1) (Si wafer) was used.
In FIG. 2, there are diffraction peaks indicating TiAlN and lower diffraction peaks indicating alumina in the y crystal structure (hereinafter, referred to as “γ-alumina peak”), but the diffraction peaks indicating alumina in the alpha crystal structure (hereinafter, referred to as “alpha-alumina peak”) are higher, suggesting that a quasi-alpha-alumina film is formed on the TiAlN film. FIG. 3, wherein the alpha-alumina peaks are higher, also indicates that a quasi-alpha-alumina film is formed also on the CrN film. In FIG. 3, peaks indicating chromium oxide formed by oxidation of the surface of the CrN film were also observed.
In contrast, there is no alpha-alumina peak observed in FIG. 4, suggesting that an alumina film mainly in the y crystal structure is formed on the Si wafer.
<Experiment B>
Then, an alumina film was formed on each of the substrates (1) to (3) under the condition same as that of the experiment A, except that the substrate temperature was 700° C. slightly lower than that in the experiment A, and the alumina film obtained was analyzed with a thin-film X-ray diffractometer. The results for the substrate (2) (TiAlN film) are shown in FIG. 5; those for the substrate (3) (CrN film), in FIG. 6; and those for the substrate (1) (Si wafer), in FIG. 7.
Although both films in FIGS. 5 and 2 were formed on the same TiAlN film, it is apparent from the Figures that an alumina film formed in a lower temperature range has a higher intensity ratio of γ- to alpha-alumina peak, or a higher content of alumina in the y crystal structure in the alumina formed, as shown in FIG. 5.
In addition, similar results were obtained when the substrate (3) (CrN film) was used. It is obvious from comparison between FIGS. 6 and 3 above, that there are in FIG. 6 γ-alumina peaks not found in FIG. 3, and that alumina in the γ crystal structure is easily formed when the film-forming temperature is in a lower temperature range.
When the substrate (1) (Si wafer) is used, there were only γ-alumina peaks observed as alumina peaks in FIG. 7, similarly to when the film-forming temperature was 750° C. (FIG. 4 above), indicating that only alumina in the γ crystal structure was formed at the film-forming temperature intended by the present invention.
After similar experiments by changing other conditions as well, the inventors have found that it was possible to form alumina in the alpha crystal structure, most easily when the substrate surface has a film containing Cr such as CrN and next easily when the substrate surface has a film containing Al such as TiAlN or Ti such as TiN or TiCN; and that it is also possible to form alumina in the alpha crystal structure, together with alumina in the γ crystal structure, even on substrates of high-speed steel or cemented carbide whereon no such a film was formed in the past. However, only alumina in the γ crystal structure was formed on a Si wafer, no matter how the condition was altered.
An object of the present invention, which was made under the circumstances above, is to provide a method of forming a quasi-alpha-alumina film not only on a substrate having a TiAlN film, CrN film, or the like formed on the surface thereof but also on a substrate such as a Si wafer, which hitherto allowed deposition only of alumina in the γ crystal structure at a temperature of approximately 800° C. or lower, at a relatively low temperature without deformation or decomposition of the substrate.
The inventors believe that it becomes possible to form a quasi-alpha-alumina film in a lower temperature range, independently of the kind of substrate, by establishing a method of forming a film on a substrate resistant to deposition of a quasi-alpha-alumina film in this manner.
Another object of the present invention is to provide a method of forming the quasi-alpha-alumina film on CrN or TiAlN that allows deposition of alpha-alumina at a lower film-forming temperature without contamination, for example, by alumina in the γ phase.